Is access fail in performance report from BSC is right?

Access fail is a key parameter of CDMA technology. Access fail cannot be measured by performance reports extracted from BSC (Base Station Controller).

We can see access fail in our performance report but it is not actual why?

MS try to access the system and send information on access channel, it is possible that due to poor RF environment BTS (Base Transreceiver System) cannot receive the request and does not acknowledge. After several try of MS access fail has been done and BTS does not aware of this as he did not have any information.

BTS only measures access fail of termination calls and access fail occur after first probe received by itself in case of origination call.

Access fail can be measured by drive test.

Communication model of CDMA

Information Stream ----------------->

Speech coding --> Channel coding --> Scrambling --> Spread Spectrum --> Modulation --> RF Transmit

<------------------------------ Information Stream

Speech decoding <-- Channel decoding <-- De-Scrambling <--De-Spread Spectrum <-- De-Modulation <-- RF Receive

 Speech coding

Speech coding is critical to digital transmission. CDMA system uses an efficient method of speech coding and extensive error recovery techniques to overcome the harsh nature of the radio channel.

The objective of speech coding is not only to maintain speech quality but also to reduce the quantity of transmitting data.

Speech coding algorithms (digital compression) are necessary to increase cellular system capacity.

Coding must also ensure reasonable fidelity, that is, a maximum level of quality as perceived by the user.

Coding can be performed in a variety of ways (for example, waveform, time or frequency domain).

Vocoders transmit parameters which control reproduction of voice instead of the explicit, point-by-point waveform description.

Variable Rate Vocoding

• CDMA uses a superior Variable Rate Vocoder which includes Full rate during speech, Low rates in speech pauses, Increased capacity and More natural sound.
•  Voice, signaling, and user secondary data may be mixed in CDMA frames. 
• The output is 20 ms frames at fixed rates:  Full Rate, 1/2 Rate , 1/4 Rate , 1/8 Rate, & Blank. 
• CRC is added to all the frames for the 13 kb vocoder, but only to the Full and 1/2 rate frames for the 8 kb vocoder.
• CRC is not added to the lower rate frames in the 8 kb vocoder, but that is ok because they consist mostly of background noise and have a higher processing gain.
• Current vocoder rates are 8kbps, 13kbps, and 8kbps EVRC (Enhanced Variable Rate Coder)

 Channel coding

Channel coding usually falls into two classes: Block interleave codes and Convolution codes. The objective of channel coding is adding additional supervising bits in the information stream to ensure get correct signal at receive side.

Convolution Encoder & Interleave Encoder

Convolution Encoder: It increases the reliability but reduce the transmitting efficiency, because each code stream adds supervising bit for rectified

Block Interleave Encoder: It does not change the efficiency but have some delays, because the transmitter and receiver must process to writing first and then reading

Scrambling

The paging channel includes many import information such as user’s IMSI, In order to keep the user’s information secret, we use the data scrambling.

Data scrambling function:

Data scrambling is accomplished by modulo-2 addition (XOR),one input is a modulation symbol(19.2ksps) coming out of the block interleaver, another input is a random sequence, which created by decimator on long code generation. That means, Use the 64 times decimator to pick up the first chip of each 64 chips to form a random sequence. So the random sequence rate is 19.2kcps. (1.2288/64)

Spread Spectrum

In CDMA we use Spread code rate: 1.2288Mcps

Following codes are used to make Spread code

• Forward Link: Walsh code
• Reverse Link: Long PN code

Direct Sequence Spreading

Output of the randomizer is direct sequence spread by the long code

Each mobile station spreads its reverse traffic channel using the same long PN code but with a different offset, which is determined by a unique 42-bit mask.

The mobile station can use one of two unique long code masks:

• A public long code mask based on the ESN
• A private long code mask

Orthogonal Spreading

• Each symbol output from the Mux is exclusive XORed by the assigned Walsh function
• Walsh function has fixed chip rate of 1.2288 Mcps
• Channels are distinguished from each other by Walsh function
• Bandwidth used greatly exceeds source rate

Modulation

QPSK & OQPSK are the types of modulation

The forward traffic channel is combined with two different PN sequences: “I” and “Q”

Baseband filtering ensures the waveforms are contained within the 1.25 MHz frequency range

The final step is to convert the two baseband signals to radio frequency (RF) in the 800 MHz or 1900 MHz range

Quadri-Phase Shift Key (QPSK) Modulation

• BASEBAND: The total frequency band occupied by the aggregate of all the information signals used to modulate a carrier
• FILTER: Electronic circuit devised to modify the frequency distribution of a signal spectrum
• BASEBAND FILTER: filter (used in quadrature modulation) that limits the input signal to the SyQuest band +-T/2, where T is the transmitted pulse rate.
• GAIN CONTROL: the gain of the overhead channels (pilot, sync and paging) in the composite I and Q is set. The gain of each forward traffic channel is constantly adjusted by the reverse link power control process.

OQPSK

The reverse traffic channel data after direct sequence spreading is spread in quadrature by adding modulo-2.This stream with the zero-offset I and Q PN short code sequences is used on the forward CDMA channel.

Why a half chip delay in the Q Component?

The data spread by the Q PN short code sequence is delayed by half a PN chip time, 406.901 ns, with respect to the data spread by the I PN short code sequence. This prevents the I and Q to change value simultaneously, thus eliminating diagonal transitions

What is Ec/Io, How it is varied with traffic, Why Ec/Io is always negative

This is a key parameter of CDMA technology measure at mobile station. It shows signal strength of each sector individually. Now question is arises in our mind that signal strength can be measured with RSSI also then why Ec/Io is used because RSSI is sum of signal strength of all sectors received by mobile. To judge signal of each sector Ec/Io parameter is used. That’s why Ec/Io is used to decide handoffs.

Ec/Io is the ratio of pilot power to total power. Total power includes pilot channel power, paging channel power, sync channel power and traffic channel power.

Let’s consider following situation. Here pilot power is 2W and total power is 5.6W. Then Ec/Io is (2/5.6) and it is equal to -4 db (equation is 10* log (2/5.6)).

Traffic channel	2W	Io
Sync channel	1.4W
Paging channel	.2W
Pilot channel	2W

In following situation pilot power is 2W and total power is 9.6 W. Here Ec/Io is (2/9.6) and it is equal to -7 db (equation is 10* log (2/9.6)).

Traffic channel	6W	Io
Sync channel	1.4W
Paging channel	.2W
Pilot channel	2W

Why Ec/Io is always negative?

We can see that Ec/Io is ratio of pilot power to total power (including noise). Now pilot power is always less then total power and ratio value of these will always less than 1. Logarithm of any less than 1 value is always negative. This is the reason why Ec/Io is always negative.

Power control methods in CDMA

Power control are two types: Forward link power control and reverse link power control

Forward link power control: 

Forward link power control is a loop control and the controlled target is the transmit power of base station. Mobile station plays assistant role.

Fast power control can divide into outer loop power control and closed loop power control. This is a slow process and less efficient.  Both loop work together. The base station continually and slowly decreases power to each mobile station (each user forward traffic channel). MS determine FER and based on that forward traffic channel decrease or increase.

Reverse Link power Control:

Reverse link power control is more accurate. It control mobile transmit power and base station plays a assistant role. It included three types of power control:

1)      Open loop power control

2)      Close loop power control

3)      Outer loop power control

1). Reverse link Open loop power control

Reverse open loop power is mobile station controlling it’s transmit power. Estimating how strong the mobile station should transmit based on a coarse measurement of how much power it is receiving from the base station. Problem in open loop control is that It Assumes same exact path loss in both directions; therefore, cannot account for asymmetrical path loss and Estimates are based on total power received; therefore the power received from other cell sites by mobile station introduces inaccuracies. It uses formula Tx+Rx-Tx_adj = -73dBm.

2). Reverse link close loop power control

It compensates for asymmetries between the forward and reverse paths.

It Consists of power up (0) & power down (1) commands sent to the mobile stations, based upon their signal strength measured at the Base Station and compared to a specified threshold(set point).

Each command requests a 1dB increase or decrease of the mobile station transmit power.

It transmitted 800 times per second, always at full power.

It allows compensating for the effects of fast fading.

3). Reverse link outer loop power control

Base station controller (BSC) determines FER on reverse traffic channel and accordingly set point threshold value is changed.  It will increase reverse capacity and improve voice quality.

Why power control required in CDMA system

CDMA is an interference limited system based on the number of user. Here each user is a noise source on shared cannel. Due to this CDMA system practically has a limit of users who can sustain, this is called soft capacity limit.

Near Far effect is basic feature of CDMA. If we assume that mobile transmit power is same for all user then mobile user near a cell jams a user that is distant from the cell. This problem may be present despite high processing gain. So an effective method to eliminate the near-far effect is necessary.

CDMA system uses power control technique to keep each MS at absolute minimum power level necessary to ensure acceptable service quality. Power control is essential for adjusting BTS and MS transmit power instantly according to communication distance (to overcome near far effect).  Ideally the power received at the base station from each mobile station should be same (Minimum signal to interference).

CDMA Frequency Band and calculation

CDMA technology is available is 450 Mhz, 800 Mhz and 1900 Mhz band. Out of these we are using 800 Mhz in current system.

The band is having two electromagnetic channels. One for Base Station to Mobile Station communication (called the FORWARD LINK or the DOWN LINK) and another for Mobile Station to Base Station communication (called the REVERSE LINK or the UP LINK).

In 800 MHz Cellular these two duplex 1.25 MHz bands are 45 MHz apart

In 1900 MHz PCS these two duplex 5 MHz bands are 80 MHz apart

In 450MHz, they are 10MHz apart

CDMA Frequency calculation

450MHz

BS receiver(Uplink): 450.00+0.025(N-1)

BS sender(downlink): 460.00+0.025(N-1)

800MHz

BS receiver(Uplink): 825.00+0.03N

BS sender(downlink):870.00+0.03N

1900MHz

BS receiver(Uplink): 1850.00+0.05N

BS sender(downlink):1930.00+0.05N

Additional Channel is CDMA1x RTT system.

Here fundamental channels pilot, paging, sync, traffic, access channel are same as CDMA IS95 which are described in previous post. In CDMA 1x RTT few channels are introduced which are described below:

1). Forward Quick Paging Channel(F-QPCH)

2). Forward Supplemental Channel (F-SCH)

3). Reverse Pilot Channel (R-PICH)

4). Reverse Supplement Channel(R-SCH)

1). Forward Quick Paging Channel(F-QPCH)

Mobile monitors incoming calls on paging channel. The mobile station continuously monitors the Paging Channel on both slotted and non slotted mode. The mobile station can “sleep” or reduce power consumption (for the power conservation) during non-active states (during the slots when the paging channel is not being monitored).

Since battery consumption is high in paging channel mode so QPCH channel (quick paging channel) is introduce to save battery of mobile. Mobile monitors QPCH to determine if there is a paging forthcoming on paging channel in its slot (looks at 1-bit paging indicator). If no flag, then mobile continues to sleep; if have flag, the mobile monitor appropriate slot and decodes general page message

Without QPCH, mobile must monitor regular paging channel slot and decode several fields to determine whether page is for it or not; this drains mobile batteries quickly.

2). Forward Supplemental Channel (F-SCH)

F-SCH Assigned for high-speed packet data (>9.6 kbps) in the forward direction; (FCH is always assigned to each call). Up to 2 F-SCH can be assigned to a single mobile. SCH cannot exist without having a fundamental channel established.

3). Reverse Pilot Channel (R-PICH)

It is used to Implement Quick Power Control on the Forward Link. It Allows base station to do timing corrections without having to guess where mobile is (in search window). Mobile can transmit at lower power, reducing interference to others. The Reverse Pilot Channel is unmodulated spread spectrum signal used to assist the base station in detecting a mobile station transmission.

4). Reverse Supplement Channel(R-SCH)

R-SCH is used for high-speed packet data (>9.6 kbps) in reverse direction. Difference between F-SCH and R-SCH is in Walsh code based spreading. F-SCH supports Walsh code lengths of 4 to 128 (1xRTT) or 1024 (3xRTT) depending on data rate and chip rate. R-SCH uses either a 2-digit or 4-digit Walsh code; rate matching done by repetition of encoded and interleaved symbols. Walsh code allocation sequence is pre-determined and common to all mobiles. Users are differentiated using long PN code with user mask.

Physical channels in CDMA and their Description

In CDMA IS95 we have some physical channel to transmit codes. BTS and mobile communicate via these channels. Physical channel is known by fundamental channel or logical channels. There are several channels in both forward and reverse direction.

Forward Channel: Pilot channel, Paging Channel, Sync Channel, Forward Traffic channel

Reverse Chanel: Access Channel, Reverse traffic Channel

Pilot Channel: It obtains a phase offset by short PN sequence to identify different base station. It has timing information. It is used to assist handoff. Mobile station use pilot strength comparison between pilots of base station to identify and perform handoff.

Sync Channel: It carries a data stream of essential system identification and parameter information used by mobiles during system acquisition stage like Pilot number, System time, Long code and paging rate (PRAT).

Paging Channel: it notify mobile station that they are receiving a call. It transmits information at a fixed rate 9600 or 4800 bps, as specified by the “PRAT” parameter sent in sync channel message. It carries system configuration parameter message, access parameters message and CDMA channel list message.

Forward Traffic Channel: It is used to transmit user and signaling information to a specific mobile station during a call.

Access Channel: Initiate communication with the base station not yet in a Call (such as transmit registration requests, call setup requests/origination message) in reverse direction. It is used to respond paging channel message.

Reverse Traffic Channel: It is used when call is in progress state to send voice traffic from subscriber, response to command from base station, request to base station.

What is difference between dBi and dBd

In CDMA, GSM technology we use antenna system to transmit and receive signals from mobile and from BTS. One of the characteristic of antenna is antenna gain. dBi and dBd is a unit to measure antenna gain with respect to isotropic antenna and dipole antenna respectively.

Dipole has 2.15 dB gain compared to an isotropic antenna, hence

dBi = 2.15 dB + dBd

For example an omni antenna has antenna gain 12.15 dBi or 10 dBd

What is difference between dB, dBm and dBW

We need to express power levels in some terms weather it is CDMA technology or GSM technology. for that we use dB, dBm and dBW as unit of power levels which are differentiate with reference power level.

dB is a relative measurement of two different power levels.

dB = 10 Log (P1/P2)

If we take reference power is 1W then it is dBW

dBW = 10 log(P1/1 W)

If We take reference power is 1mw then it is dBm

dBm = 10 Log(P1/1 mw)

Normally we use dB when expressing the ratio between two power values and dBm when expressing an absolute value of power.